Police have collected forensic evidence from the store but lack video surveillance and have not formed a solid theory about why the attack happened. But they think the attack was random and have found no evidence that either victim knew the attackers.

“We have no indication at this point that this was anything but a random crime of opportunity,” Montgomery County Police Chief J. Thomas Manger said Monday, as officials announced a $136,000 reward for help in solving the case.

Manger also provided a more exact time line of the slaying than had been released previously:

At 9 p.m. Friday, Murray, 30, and a fellow employee closed the Bethesda Row yoga store Lululemon Athletica. At 9:45 p.m., they set the alarm, locked the store and left.

A short time later, one of the women realized she had left something behind, called her co-worker and the two met back at the store. They entered the front door at 10:05 p.m., Manger said.

“We believe that the two suspects in this case followed them in just seconds after the two victims entered the store,” he said.

The men wore dark clothing, gloves and ski masks, Manger said. One is thought to be 6 feet tall, the other 5 feet 3.

Both women — one age 27, the other 30 — were violently attacked. Murray received “catastrophic” injuries that rattled even longtime law enforcement officers, sources who spoke on the condition of anonymity because the investigation is ongoing said.

The surviving woman was discovered Saturday morning after another employee came to open the store. It is unclear whether she couldn’t move because she was so tightly bound, unconscious or both. She was hospitalized and her condition continues to improve, police said.

The store is adjacent to an Apple retail outlet, which began selling its hot new iPad 2 about 5 p.m. Friday. That has led some to speculate that the assailants wanted to be near the store at closing time, 9 p.m., to rob customers going to their vehicles.

But the sources said they’ve uncovered no evidence to support the theory. The draw of many well-heeled Apple customers might be a coincidence, they said.

Detectives are hopeful that forensic evidence from the crime scene will lead to DNA profiles. “We have a team of detectives that are working around the clock to try to solve this case,” said Manger, adding that police are keeping an open mind to new developments.